This info is specifically from a GTR, but I believe the wiring is common with the GTO.
There should be two black wires attached to the negative side of the battery. One goes to the black terminal of the rectifier, the other goes to the bolt which attaches the rear of the voltage regulator to the frame. There is also a black/white wire attached to the negative terminal of the battery which comes from the alternator.
On the rectifier there are five terminals:
1. Red terminal has two red wires: One goes directly to the battery, the other goes through the fuse and into the main wiring harness.
2. Black terminal has one black wire, goes directly to the battery.
3. Yellow terminal has one yellow wire from the alternator.
4. Pale blue terminal has one pale blue wire from the alternator.
5. White terminal has one white wire, from the main wiring harness.
Hope this and the pictures help.